This invention relates to a space-diversity radio receiver in which a receiver output signal is produced after intermediate frequency signals derived from two or more signals received through different transmission paths are combined and which is for use in a broad-band digital radio communication system. The receiver is usable even when frequency diversity is combined with space diversity to provide hybrid diversity and in either a repeater station and a terminal station of the system.
In a microwave communication system in general, most distances between adjacent stations are from 30 to 50 kilometers long in view of economy of the system and because of convenience of site selection for the stations. Selective fading, to be described later in detail, often appears when the signals transmitted through the system are frequency modulated, particularly in case where the section is relatively long and extends either along an even topography or a seashore. Resulting from a multipath effect according to which a modulated carrier signal is received at a receiver through a direct path and at least one curved path variable with the atmosphere conditions, the selective fading deteriorates the transmission performance and, even worse, may result in interruption of communication.
Space diversity in which the modulated carrier signal is received by two or more antennas for a receiver as a plurality of receiver input signals, respectively, is effective in ensuring highly reliable communication in a section where the selective fading is liable to occur. This is because it is seldom that electric field intensities of the respective signals thus received through a plurality of transmission paths, each comprising a direct and a curved path, are simultaneously reduced to an objectionable extent. In order to derive a receiver output signal, the receiver comprises a combining circuit for the receiver input signals either in a carrier frequency stage or in an intermediate frequency stage. The signal combining circuit may be a switching combiner for combining the receiver input signals with that of the higher electric field intensity selected, a linear combiner, or a ratio squarer for minimizing the signal-to-noise ratio of the receiver output signal.
In a digital microwave communication system, the selective fading results in distortion of the modulated carrier signal waveform as will later be described with reference to a few of the figures of the accompanying drawing. This, in turn, gives rise to bit errors and deteriorates the bit error rate. In a 6-GHz FDM-FM (frequency division multiplexed-frequency modulated) communication system, the total duration of fading in a section was one hour per year. By adoption of diversity reception, the total duration was reduced to about one minute per year. This corresponds to an improvement factor of sixty. This great improvement factor is achieved because the effect of increase resulting in thermal noise from fading is more dominant than the waveform distortion in the FDM-FM system.
It has recently been confirmed that the bit error rate is more seriously deteriorated by the waveform distortion than by the effect of the thermal noise increment in a broad-band multilevel digital radio communication system, such as the PCM-8PSK (pulse code modulated-eight-level phase shift keying) system or the PCM-16QAM (quadrature amplitude modulation) system. For example, the improvement factor of the bit error rate is only six in a 78-Mbit/s 8PSK space diversity communication system according to an article contributed by William T. Barnett to IEEE Transactions on Communications, Vol. COM-27, No. 12 (December 1979), page 1842-1848, under the title of "Multipath Fading Effects on Digital Radio." The improvement factor is seven in a 200-Mbit/s 16QAM system according to another article contributed by Shozo Komaki et al to the same issue, pages 1854-1861, and entitled "Characteristics of a High Capacity 16 QAM Digital Radio System in Multipath Fading." The improvement factor is eighteen in a 91-Mbit/s QPRS (quadrature partial response signalling) system according to still another article contributed by Carl W. Anderson et al to the same issue, page 1870-1875, and entitled "The Effect of Selective Fading on Digital Radio."
The poor improvement factors hitherto attained, show the importance of getting rid of the waveform distortion in the space-diversity broad-band digital radio receiver. As suggested in the articles referred to hereinabove, the waveform distortion depends on an amplitude dispersion caused by the selective fading. No receivers, however, have been developed to reduce the amplitude dispersion.